A demand for multilayer semiconductor elements has recently increased following the increase in the degree of integration of semiconductor integrated circuits and the rise of element density. Because spacing between wirings further decreases as the degree of integration of semiconductor integrated circuits grows, wiring delay caused by the increase in capacitance between the wirings becomes a problem. Here, the wiring delay (T) is represented by the following formula: T∝CR and is, therefore, affected by a wiring resistance (R) and a capacitance (C) between wirings. The relationship between a dielectric constant (∈) and the capacitance (C) between wirings is represented by the following formula C=∈0∈r·S/d, where S is an electrode surface area of an electrode, ∈0 is a dielectric constant of vacuum, ∈r is a dielectric constant of an insulating film, and d is a distance between wirings. Although the capacitance (C) between wirings can be decreased by reducing the wiring thickness and decreasing the electrode surface area, the decrease in wiring thickness causes additional increase in wiring resistance (R), thereby making it impossible to attain a high speed. Therefore, decreasing the dielectric constant of the insulating film is an effective means for reducing the wiring delay (T) and increasing speed.
There is a trend to decrease a spacing between metal wirings in semiconductor devices having a multilayer wiring structure. Accordingly, the impedance of metal wiring determined by electrostatic inductance increases, and the increase in delay of response speed and power consumption causes concerns. Therefore, it is necessary to reduce the specific permittivity of interlayer insulating film provided between a semiconductor substrate and a metal wiring or between metal wirings as much as possible.
Inorganic materials such as silicon dioxide (SiO2), silicon nitride (SiN), and phosphosilicate glass (PSG), and organic polymer materials such as polyimides have been used as conventional materials for insulating films.
However, a dielectric constant of CVD-SiO2 films that are widely used in semiconductor devices is as high as about 4. Further, a SiOF film that has been studied as a CVD film with a low dielectric constant has a dielectric constant of about 3.3 to about 3.5, but it has high hygroscopicity and the dielectric constant thereof increases with time.
It has recently been suggested to add an organic resin or the like that is evaporated or decomposed on heating to a material for forming a film with a low dielectric constant and to obtain a porous film in which pores are formed by heating during deposition. Because porous films have pores, a dielectric constant lower than that in the conventional configurations can be realized, but the problem that is presently associated with this approach is that the pore size is as large as 10 nm or more and where a porosity (pore presence ratio) is increased with the object of further decreasing the dielectric constant, moisture absorption causes increase in dielectric constant and decrease in film strength.
At present, a film is formed by applying a chemical solution so as to reduce a film density for the purpose of lowing the dielectric constant of the film, and as the chemical solution, those using a silicon compound have been known (see Japanese Patent Application Laid-Open (JP-A) No. 2001-127152). However, in the case where the silicon compound, especially polycarbosilane compound is used, there is a problem such that formation of a film is difficult. This is because the structure of the polycarbosilane compound is expressed by the following formula: (—Si—R-)n-, where R is an organic group containing at least one C, and has a similar structure to that of a hydrocarbon material in view of the environment for bonding, and thus the crosslinking does not progress. Therefore, it has been found that the obtained film is insufficient in terms of the strength thereof to serve as an insulating film used for a semiconductor device.